In order to address the dynamics of DNA topology in hyperthermophilic archaea, we analysed the topological state of several plasmids recently discovered in Thermococcales and Sulfolobales. All of these plasmids were from relaxed to highly positively super-coiled in vitro, i.e. they exhibited a significant linking excess compared to the negatively supercoiled plasmids from mesophilic organisms (both Archaea and Bacteria). In the two archaeai orders, plasmid linking number (Lk) decreased as growth temperature was lowered from its optimal value, i.e. positively super-coiled plasmids were relaxed whereas relaxed plasmids became negatively supercoiled. Growth temperatures above the optimum correlated with higher positive supercoiling in Sulfolobales (Lk increase) but with relaxation of positive supercoils in Thermococcus sp. GE31. The topological variation of plasmid DNA isolated from cells at different growth phases were found to be species specific in both archaeai orders. In contrast, the direction of topological variation under temperature stress was the same, i.e. a heat shock correlated with an increase in plasmid positive supercoiling, whilst a cold shock induced negative supercoiling. The kinetics of these effects were analysed in Sulfolobales. In both temperature upshift (from 80 to 85C) and downshift (from 80 to 65C), a transient sharp variation of Lk occurred first, and then DNA supercoiling progressively reached levels typical of steady-state growth at the final temperature. These results indicate that DNA topology can change with physiological states and environmental modifications in hyperthermophilic archaea.